Optimisation of the Current Ramp for the LHC

The field quality of the main magnets in the LHC will be, in part, dependent upon the shape of the magnetic field as a function of time. A theoretical optimisation of this function has been carried-out with the aim of minimising the dynamic errors [3]. This work resulted in the definition of a current ramp function composed of mathematically defined, smoothly joining segments. A prototype digital controller based on a DSP has been developed and built [4]. In this equipment the current ramp is computed from the segment equations in real time. The user need only supply the characteristic parameters for the segments in order to define the ramp. In this paper, the effect of the ramp function on the error terms is discussed and the corresponding segment equations are given. The prototype implementation is described and actual results are shown

Developments in the High Precision Control of Magnet Currents for LHC

The LHC will require over 1700 magnet power converters, some of which will need an unprecedented precision of about 1 ppm (of 13 kA). This paper presents the approach taken, prototype methods, initial results and charts future design directions. These results confirm that such performance can be obtained reliably and at a reduced cost compared to conventional methods. Developments of a real-time controls infrastructure needed to support on-line beam feedback are outlined

Evolution in controls methods for the SPS power converters

In common with much accelerator specific material, there is a constant need to improve both hardware and software for power converter control. Maintenance and performance improvements of older systems have become extremely tedious and in some areas impossible. By using modern real-time software and the latest high-performance processors, such problems should be substantially reduced. This paper describes the software concepts and the hardware chosen for the upgrade of the existing facilities. Using the UNIX compatible LynxOS real time kernel, running on a PowerPC 603 in a VME environment, this new approach provides excellent performance while retaining the desired flexibility for future enhancements. The 64 channel system is implemented as a set of cooperating processes, several of which are multi-threaded. Processes include analogue function generation, analogue measurement and digital I/O, all of which are accurately scheduled by the accelerator timing system. This generalised structure, which performs complex sequences of operations is described in detail, as well as how it can be adapted to a wide variety of accelerator tasks

A Strategy for Controlling the LHC Magnet Currents

The LHC will require an unprecedented precision of a few ppm in the control of the current in the major magnetic circuits. As a result of the optimisation of the machine design, the machine will be powered in eight sectors with separate power converters. This scheme, along with other operational constraints, has led to a re-evaluation of the methods needed to ensure adequate performance. An overview of the strategy envisaged to meet this new challenge is presented, along with details of digital control and correction methods, new techniques for analogue to digital conversion and improvements in DC current transducers above 10 kA

High-current performance evaluation of DCCTs

The evaluation of high performance DCCT's to the ppm level has never been an easy task. With the LHC demanding currents up to 13 kA, a whole series of problems has arisen in the accurate measurement of these devices. In order to tackle these problems, new facilities have been designed for laboratory measurements under full power operating conditions. These include a high performance low voltage 20 kA power converter, quasi-coaxial bus-bar structures, Kusters Bridge range extenders and a novel bipolar 0 - 10 A current calibrator with resolution and linearity better than ± 0.5 ppm. This paper will present an overview of the complete facility and give more detail on the new current calibrator. Initial results will be presented, along with application areas which advance the state of the art in this field of measurements

The All-digital Approach to LHC Power Converter Current Control

The design of the LHC machine imposes severe demands upon the control of current in the 1700 magnet circuits. This has required the use of novel methods for the control of individual power converters and of the magnet current control system as a whole. This paper will review the chosen hardware and software methods and architectures. The digital regulation techniques used to achieve the overall targets for short-term stability (<3ppm) and reproducibility (<10ppm) of the 24 principal LHC circuits will be discussed. While the proposed system architecture will follow the canonical three-layer design, so successfully exploited in LEP, the software will be far from traditional. This software must be more reliable and maintainable than ever before, and will need to integrate with advanced object-oriented applications via commercial middleware. These challenges will be faced by applying object-oriented techniques throughout the system and by harnessing the power of XML for system definition

Dynamic Effects and their Control at the LHC

Tune, chromaticity and orbit of the LHC beams have to be precisely controlled by synchronising the magnetic field of quadrupole, sextupole and corrector magnets.This is a challenging task for an accelerator using superconducting magnets, whose field and field errors will have large dynamic effects.The accelerator physics requirements are tight due to the limited dynamic aperture and the large energy stored in the beams.The power converters need to be programmed in order to generate the magnetic functions with defined tolerances. During the injection process and the energy ramp the magnetic performance cannot be predicted with sufficient accuracy, and therefore real-time feedback systems based on magnetic measurements and beam observations are proposed. Beam measurements are used to determine a correction factor for some of the power converters. From magnetic measurements the excitation of small magnets to compensate the sextupolar (b3) and decapolar (b5) field components in the dipole magnets will be derived. To meet these requirements a deterministic control system is envisaged

The LHC Prototype Full-Cell: Design Study

As a continuation of the experimental program carried-out with String 1, project management decided toward the end of 1995 to construct an LHC prototype Full-Cell, also known as String 2. The present document reports on the outcome of the one-year design effort by the community of specialists contributing to the LHC Prototype Full-Cell: it informs specialists on the boundary areas with other syste ms and conveys to the general public a description of the facility